The present invention relates to rotary fluid pressure devices such as low-speed, high-torque gerotor motors, and more particularly, to a novel valving arrangement for such motors, which provides both improved volumetric efficiency and improved mechanical efficiency.
Low-speed, high-torque gerotor motors of the type to which the present invention relates are typically classified, in regard to their method of valving as being either "spool valve" motors or "disc valve" motors. As used herein, the term "spool valve" will refer to a generally cylindrical valve member in which the valving action occurs between the cylindrical outer surface of the spool valve and the adjacent cylindrical surface of the surrounding housing member. In a typical spool valve motor of the type produced commercially by the assignee of the present invention, the spool valve is integral with the motor output shaft (see U.S. Pat. No. 4,592,704, assigned to the assignee of the present invention).
In the typical spool valve motor, side loads (loads exerted radially on the output shaft) are transmitted to the spool valve, requiring that the spool valve include one or more bearing or journal surfaces able to engage the cylindrical spool bore defined by the housing. Such bearing surfaces add to the overall size, complexity, and machining cost of the spool valve. Partly as a result of the presence of such bearing surfaces, the spool valve motor is subject to thermal shocks, i.e., warm hydraulic fluid entering a cold motor can cause expansion of the spool valve, and thermal seizure of the spool within the bore. Also, as will be understood by those skilled in the art, it is obviously not possible with a spool valve motor to offer the customer a "bearingless" version in which the dogbone shaft transmits torque directly from the gerotor gear set into the customer's internally splined device (such as a wheel hub).
The correct valve timing of a spool valve motor is dependent upon the correct rotational relationship between the spool valve and the gerotor ring (which defines the volume chambers). The spool valve is driven by the dogbone shaft, which transmits torque from the gerotor to the output shaft. Therefore, any wear of the torque transmitting spline connection (either between the star and the dogbone or between the dogbone and the output shaft) changes the timing of the spool valve.
One final disadvantage of the typical spool valve motor, as it relates to the present invention, is the tendency for the volumetric efficiency of a spool valve motor to decrease drastically with increasing pressure. It has been determined that the spool valve in a typical spool valve motor may undergo a diametral "collapse" or reduction in overall diameter, of approximately 0.001 inches when the motor is subjected to an operating pressure differential of approximately 2,000 psi. Any such collapse of the spool valve results in an increased radial clearance between the spool valve outer surface and the spool bore, permitting cross-port leakage between adjacent high-pressure and low-pressure regions, and substantially reduced volumetric efficiency.
One of the primary advantages of a spool valve motor is that an almost negligible amount of the motor output torque is used merely to drive the spool valve. Thus, the typical spool valve motor has a relatively high mechanical efficiency.
Accordingly, it is an object of the present invention to provide an improved low-speed, high-torque gerotor motor which retains the high mechanical efficiency characteristic of the typical spool valve motor, but overcomes the various disadvantages of spool valve motors.
It is a more specific object of the present invention to provide an improved spool valve motor which substantially overcomes the problem of pressurized collapse of the spool valve, and thus has a significantly better volumetric efficiency than prior art spool valve motors.
A "disc valve" motor as used herein shall mean a motor in which the valve member is generally disc-shaped, and the valving action occurs between a transverse surface of the disc valve (perpendicular to the axis of rotation) and an adjacent, stationary transverse surface (see U.S. Pat. No. 3,572,983, assigned to the assignee of the present invention, and incorporated herein by reference).
The typical disc valve motor produced by the assignee of the present invention has been relatively more expensive to produce than a similar spool valve motor. One reason for the greater expense is that a disc valve motor requires some sort of axial pressure-balancing mechanism which, in the motors produced commercially by the assignee of the present invention, actually provides a pressure "overbalance", i.e., a net force biasing the disc valve against the stationary valve surface. If the disc valve were truly axially balanced, "lift-off" of the valve member (i.e., axial separation of the disc valve from the stationary valve) would occur readily, resulting in substantial cross-port leakage and stalling of the motor. However, lift-off of the disc valve is largely prevented by the pressure overbalance of the balancing mechanism.
One major disadvantage of the typical disc valve motor is a result of the necessary pressure overbalance applied to the disc valve, as described above. The overbalance force, biasing the disc valve into sliding, sealing engagement with the adjacent stationary valve surface, results in lower mechanical efficiency in disc valve motors because the torque required to drive the disc valve detracts from the net torque output of the motor.
One of the primary advantages of disc valve motors is that, because of the sealing engagement between the disc valve and the stationary valve surface, the volumetric efficiency of the motor decreases only very slightly with increasing pressure differential across the motor.
Accordingly, it is an object of the present invention to provide an improved low-speed, high-torque gerotor motor which maintains the good volumetric efficiency characteristic of the typical disc valve motor, while overcoming the disadvantages of disc valve motors.
The above and other objects of the present invention are accomplished by the provision of a rotary fluid pressure device of the type including housing means defining fluid inlet and fluid outlet means. A fluid energy-translating displacement means is associated with the housing and includes one member having rotational movement relative to the housing and one member having orbital movement relative to the housing, to define expanding and contracting fluid volume chambers in response to the rotational and orbital movements. A valve means cooperates with the housing to provide fluid communication between the fluid inlet and the expanding volume chambers and between the contracting volume chambers and the fluid outlet. The device includes an input-output shaft and means for transmitting torque between the member of the displacement means having rotational movement and the input-output shaft. The valve means comprises a generally cylindrical spool valve member, defining a pair of end surfaces, and defining valving passages on its outer cylindrical surface. The spool valve is rotated at the speed of rotation of the member of the displacement means having rotational movement. The housing means comprises a valve housing section defining a spool bore and surrounding the spool valve member, and further defining a plurality of meter passages, each being in fluid communication with one of the fluid volume chambers.
The improved rotary fluid pressure device is characterized by the spool valve member and the valve housing section being disposed on the side of the displacement means which is opposite the input-output shaft. The spool valve member is relatively solid, whereby the spool valve member is able to withstand the force of a predetermined fluid pressure, without substantial collapse of the spool valve member.
The improved rotary fluid pressure device is further characterized by the valve housing section including a relatively thicker outer housing portion and a relatively thinner inner housing portion defining the spool bore. The inner housing portion is press-fit within the outer housing portion with an interference fit sufficient to preload the inner housing portion with a preload force at least equal to the equivalent force of the predetermined fluid pressure, whereby the inner housing portion will be able to withstand the predetermined fluid pressure, without substantial expansion of the spool bore.